This invention relates to a process for preparing a water-in-oil (W/O) emulsion of a water-soluble polymer. More particularly, it relates to a process for preparing a W/O emulsion in a very stable manner while inhibiting the formation of agglomerates which accompanies the preparation.
Water-soluble polymers typically including polyacrylamide and copolymers mainly comprising acrylamide have been widely used in various fields as, for example, flocculating agents, thickeners, wet strength agents, electrolytic refining agents, and chemicals for petroleum recovery.
These water-soluble polymers are generally supplied in the form of a powder, a gel, or a W/O emulsion. The W/O emulsion can be dispersed or dissolved in an aqueous medium more rapidly and more easily than the former two types of products and has therefore been the focus of more attention.
However, preparation of such a W/O emulsion is accompanied by formation of agglomerates in a polymerization apparatus comprising a polymerization vessel and connected circulating line, etc., which has made the production process complicated and reduced production capacity. Namely, the agglomerate attached to the inner wall of a polymerization apparatus causes operational disorders, such as reduction in cooling efficiency, and removal of the agglomerates requires a great deal of time and labor, which results in a reduction in operating efficiency for production. Moreover, formation of agglomerates reduces the effective content of the water-soluble polymer in the emulsion. All these problems result in an increase in production cost.
The present invention seeks to solve these problems arising from formation of agglomerates, thus making a considerable contribution, particularly in production on an industrial scale.
In order to overcome the above-described problems, the inventors have conducted extensive investigations from various aspects, such as emulsion compositions, surface active agents and other additives, and stirring conditions. As a result, it has been surprisingly found that control of the dissolved oxygen concentration in an emulsion during polymerization to about 100 ppb or below is extremely effective to suppress formation of agglomerates.
The above and other objectives and advantages are obtained by the present invention which relates to a process for preparing a W/O emulsion of a water-soluble polymer comprising polymerizing at least one water-soluble vinyl monomer in a W/O emulsion, wherein the dissolved oxygen concentration of the emulsion under polymerization is about 100 ppb or less.
The W/O emulsion prepared by the process according to the present invention comprises an aqueous phase comprising at least one water-soluble polymer in the form of colloidal particles or droplets dispersed in an oily phase comprising a hydrophobic liquid and a W/O emulsifying agent.
Water-soluble vinyl monomers which can be used to obtain the water-soluble polymer used in the present invention, for example, include acrylamide, methacrylamide, 2-acrylamido-2-methylpropanesulfonic acid (or a salt thereof), acrylic acid (or a salt thereof), dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, dimethylaminohydroxypropyl (meth)acrylate, dimethylaminoethyl acrylamide, quaternary ammonium salts of these cationic monomers (e.g., methacryloyloxyethyltrimethylammonium chloride), and vinylpyrrolidone. These monomers may be used either individually to give homopolymers, or in combination of two or more thereof to give copolymers or higher polymers.
As noted above, the water-soluble vinyl monomer is in an aqueous (discontinuous) phase in the form of colloidal particles or droplets. Such colloidal particles are dispersed in an oily or continuous phase, described below.
Hydrophobic liquids which can be used in the oily phase of the W/O emulsion include liquid hydrocarbons and substituted liquid hydrocarbons. Examples of preferred hydrophobic liquids are halogenated hydrocarbons, e.g., perchloroethylene, and aromatic or aliphatic hydrocarbons, e.g., dodecane, tetradecane, benzene, xylene, kerosene, and liquid paraffin, with aliphatic hydrocarbons being the most preferred.
Emulsifying surface active agents which are preferably used as agents for emulsion formation in the oily phase of the W/O emulsion include those having a hydrophiliclipophilic balance (HLB) of from 1 to 10, and more preferably from 2 to 6, e.g., sorbitan monooleate, sorbitan monostearate, polyoxyethylene sorbitan monooleate, polyoxyethylene oleyl ether, polyoxyethylene nonylphenyl ether, and mixtures thereof.
The proportion of the aqueous phase in the emulsion preferably ranges from about 50 to 90% by weight, and more preferably from 55 to 80% by weight, based on the total weight of the emulsion. The amount of the monomer(s) in the emulsion preferably ranges from about 15 to 80% by weight, and more preferably from 20 to 70% by weight, based on the total weight of the emulsion.
In order to obtain a stable emulsion, the emulsifying surface active agent is preferably used in an amount of from about 1.0 to 20.0% by weight, and more preferably from 2.0 to 15.0% by weight, based on the total weight of the hydrophobic liquid.
The W/O emulsion of the present invention can be obtained by mechanically agitating a mixture of the above-described components by means of, for example, a Waring blender.
After preparation of the W/O emulsion containing the water-soluble vinyl monomer(s) as described above, inhibiting the formation of agglomerates during polymerization is accomplished by reducing the dissolved oxygen concentration of the emulsion under polymerization to an extremely low level, i.e., about 100 ppb or less, and preferably 50 ppb or less. Removal of oxygen can be achieved by blowing nitrogen or argon through the emulsion in a closed container through a gas inlet.
Production of a W/O emulsion containing a water-soluble polymer can be carried out by known techniques, such as the process disclosed in JP-B-34-10644 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined published Japanese patent applicationxe2x80x9d) by Banderhoff, et al. Of course, such known techniques do not include carrying out polymerization under conditions of reduced dissolved oxygen concentration, the essence of the present invention. In short, an aqueous phase containing at least one water-soluble monomer and a hydrophobic liquid are mixed, emulsified and dispersed with the aid of an emulsifying surface active agent, as generally discussed above. After reducing the dissolved oxygen concentration of the emulsion to about 100 ppb or less as mentioned above, the resulting W/O emulsion containing the water-soluble monomer(s) is subjected to polymerization under such reduced dissolved oxygen conditions. Banderhoff et al disclose conducting polymerization in the presence of a polymerization initiator capable of forming a free radical to thereby obtain a W/O emulsion of the water-soluble polymer, but other types of emulsion polymerization may also be employed, as long as it is carried out under the reduced dissolved oxygen conditions of the present invention.
Examples of suitable polymerization initiators include redox initiators comprising a combination of a peroxide, e.g., a persulfate and an alkyl peroxide, and a reducing agent, e.g., a sulfite, a ferrous salt and an amine compound; and azo type pyrolysis initiators, e.g., azobisisobutyronitrile, 2,2xe2x80x2-azobis (2-amidinopropane) hydrochloride, and 4,4xe2x80x2-azobis(4-cyanovaleric acid). Polymerization may also be induced by light irradiation in the presence of a photosensitizer, e.g., benzophenone and benzoin methyl ether.
These polymerization initiator and photosensitizer are respectively used in an amount usually ranging from about 10 to 5000 ppm, and preferably from 30 to 3000 ppm, based on the monomer(s).
If desired, the system undergoing reversed phase emulsion polymerization may further contain, in addition to the above-mentioned components, such commonly used components as chain transfer agents, other surface active agents for inversion, chelating agents, buffering agents, salts, and the like.
After polymerization, the W/O emulsion containing the water-soluble polymer according to the present invention can be inverted to an aqueous solution of the water-soluble polymer in a known manner, e.g., by adding the emulsion to an aqueous medium containing a surface active agent for inversion, or by adding a surface active agent for inversion to the emulsion and then adding the emulsion to an aqueous medium.
According to the present invention, a W/O emulsion of a water-soluble polymer can be prepared while considerably suppressing formation of agglomerates. The present invention is particularly advantageous for industrial production.